IR sensing and imaging is based on detecting a temperature and/or emissivity differences on the surface of an object and between the object and its surroundings. The main radiation emitted by surfaces at ambient temperature is in the LW range. For sensitive detection in the LW range the detector has to be kept at a temperature that is considerably lower than the ambient temperature and it is therefore common practice to keep the detectors at a cryogenic temperature of about 77.degree. K. For sensitive operation, detectors for the MW range should also be cooled to about 77.degree. K.
To achieve this, all state of the art IR detector devices comprise specially designed Dewars which have a socket holding a so-called cold finger generating the required cryogenic temperature and which is in thermal contact with a detector array, and a sealed chamber (detector chamber) with an IR transparent window for admitting received and focused IR radiation which houses the detectors so as to face the said IR transparent window. The detector chamber is usually evacuated and outside thereof there is provided an optical focusing system whose focus is in the plane of the detector array inside the detector chamber.
The detector chamber of the Dewar further holds a so-called cold shield or cold stop which serves as aperture and is designed to admit only the convergent IR radiation arriving from the optical focusing system. Due to the fact that the backside of the cold shield is cold, it does not emit interfering IR radiation of its own which, if it were to happen, would disturb the imaging process.
There are also known some IR detector devices with a non-evacuated detector chamber. In these devices there exists a great heat load on the refrigerator which is undesired. Moreover, the window may be extensively cooled which can lead to moisture condensation on the outer side thereof which interferes adversely with the operation. Consequently IR detector systems with non-evacuated detection chamber were found to be limited in use.
The detectors in an IR detection system of the kind specified are selected from among a small group of compounds such as Hg.sub.1-x Cd.sub.x Te (MCT), InSb, Pb.sub.1-x Sn.sub.x Te (LTT) and others. The most commonly used material for the LW range detectors is MCT. The preparation of this material and the fabrication of detector devices therefrom are done by very special techniques under most rigorously controlled conditions. However, the total production yield of detector arrays for the LW radiation range with the quality and performance required for military use, is very low and accordingly the currently used detector devices are predominantly linear arrays of photoconductive (PC) type detectors The fabrication techology of photovoltaic type (PV) detector arrays, on the other hand, is not yet mature and the production yield of such devices is extremely low. Usually too many detectors in a PV detector array are of too pure quality and such devices are not accepted for use in systems even when a fair proportion of the detectors in an array do have the required quality.
It has been recognized that PV type detector devices would be very useful because they can be connected to existing multiplexing devices for signal processing inside the detector chamber. Focal Plane Array (FPA) devices are at present the ultimate structure and comprise a mosaic, two dimensional PV detector array structure, with a monolitic or hybridic attached Charge Coupled Device (CCD) type signal processor. Such an array can record at once the entire image field of view, as required by the IR optical staring assembly. No scanning is needed in this case. The external signal processing hardwork is relatively simple and a very large number of densely arranged detectors could in principle be incorporated in such a FPA device, but as mentioned above the present production yield of such devices is much too low. Therefore at present most IR detection systems for the 8-12 micron range comprise MCT photoconductive linear detector arrays, which covers at once only a fraction of the object space, and the field of view is scanned at high speed through the object space. By one scanning method, the so-called serial scan technique, the detector array is made to scan successively adjacent strips of the object space moving during each sweep from left to right or right to left, as the case may be, and downward or upward, again as the case may be, between sweeps. By another scanning technique, the so-called parallel scanning, a one-dimensional array is scanned across the object space. In either scanning system, the interstices between the individual detectors in an array may distort the resulting image.
EP A1 0100124 discloses an IR imaging/homing system of the kind specified, in which a Dewar with an evacuated detector chamber and cooled detectors are located out of the focal plane of the optical system. While the Dewar flask is static, the optical system is swingable for scanning purposes, and this is the heart of that invention. One end of a bundle of optical fibres is located at the focal area of the optical system outside the flask and the other end of the optical fibre bundle is also located outside the Dewar flask with a lens system outside the Dewar flask focusing the emerging IR radiation on the detectors inside the detector chamber through a Dewar IR window, and it is alleged that the terminal of each optical fibre is targeted onto one particular detector of the array. As the detectors are usually very small, such alignment is quite difficult.